Digital subscriber line (DSL) and wireless systems often are susceptible to any of a variety of signal impairments, including, for example, near end cross talk (NEXT), far end cross talk (FEXT), radio frequency interference (RFI), attenuation due to long distances, and noise introduced by bridge taps. A number of techniques have been developed in an attempt to limit or eliminate some or all of these impairments. For various reasons, however, these techniques are limited in that they typically either fail to satisfactorily reduce all of the various forms of impairment or they are expensive and unwieldy to implement, or both.
One such technique developed to reduce signal impairments in asynchronous DSL (ADSL) includes the implementation of a vector or multi-input, multi-output (MIMO)-based broadband access architecture. Vector-based techniques provide for dynamic spectrum allocation among connected units and supervised matrix cancellation of central office (CO) NEXT and FEXT. Vector-based techniques, however, are expensive to implement and rely on a number of often unreasonable assumptions, such as the assumption that all customer premise equipments (CPEs) are connected to at least one other CPE, the assumption that the allocated spectrums may be quickly verified against spectral compatibility rules, and the assumption that the allocated mask may become standardized even though it often takes considerable time just to standardize a slight modification to an edge of a mask. Moreover, vector-based techniques typically do not address the reduction of impairments caused by CPE cross talk when CPEs are not interconnected, RFI, and bridge taps.
Bonding is another technique commonly used in single carrier systems, such as, for example, G.SHDL (also known as ITU G.991.2). Bonding typically comprises transmitting the symbol payload in parallel over two or more channels (e.g., two or more twisted pair lines). Diversity techniques then may be used with the bonded physical channels to improve the reach of the transmitted symbol stream or multiplexing techniques may be used to improve the rate of the symbol stream. Diversity commonly refers to the use of some form of spatial redundancy shared by multiple channels to improve the robustness of the transmitted signal whereby the same symbol is transmitted over the bonded channels and the received symbols are compared at the destination to arrive at an estimate of the actual symbol transmitted. Various techniques may be used to implement diversity, such as, for example, space-time encoding or joint detection algorithms. On the contrary, multiplexing (also commonly referred to BLAST in the wireless context) provides for the parallel transmission of a separate symbol stream over each bonded channel. The individual streams then are demultiplexed at the destination to form a single symbol stream. It will be recognized that the use of diversity techniques provides for robustness in the presence of significant signal impairments at the cost of rate while multiplexing provides for increased rate while being more susceptible to signal impairments. Conventional data transmission systems typically fail to benefit from both multiplexing and diversity because the goals and implementation of multiplexing often differ from those of diversity.